ABSTRACT Excess overall adiposity (obesity) is one of the most significant risk factors for breast cancer in postmenopausal women and is a key predictor for poor prognosis. Excess central or visceral adiposity, rather than overall or subcutaneous adiposity, is more indicative of abnormal metabolic profiles and is strongly associated with obesity- related outcomes, including breast cancer incidence and prognosis. Postulated mechanisms underlying the association between obesity and post-menopausal breast cancer include higher levels of estradiol, hypercholesterolemia, and inflammation. Laboratory studies established the cholesterol metabolite, 27- hydroxychlolesterol (27HC), as an endogenous selective estrogen receptor modulator capable of promoting breast cancer growth and metastasis. However, epidemiologic data on the role of 27HC in breast cancer incidence and tumor characteristics is lacking, particularly for diverse racial/ethnic populations. Thus, we propose to conduct a large-scale epidemiologic study to examine the associations of 27HC with body fat distribution, breast cancer incidence, and clinicopathologic characteristics among participants of the Multiethnic Cohort (MEC), which includes five major racial/ethnic groups in the U.S.?African Americans, Japanese Americans, Native Hawaiians, Latinos and Whites. In Aim 1, we will examine the association between body fat distribution and circulating levels of 27HC in 913 healthy females and conduct a longitudinal study among a subset of these women (n=796) to examine the association between waist-hip-ratio and circulating levels of 27HC. In Aim 2, we will examine the independent association of circulating levels of 27HC and joint associations of 27HC, estradiol, and lipids with postmenopausal breast cancer incidence (cases/controls=1,572/1,572). In Aim 3, we will examine in breast tumor tissue (n=700) the association between gene expression levels of 27HC-regulating enzymes and clinicopathologic characteristics, including grade, stage, PAM50-defined molecular subtypes, and breast cancer-specific gene expression signatures. The strengths of this proposal include: (1) it represents the first large-scale multiethnic population-based epidemiologic investigation of 27HC and breast cancer; (2) it investigates the possible pathway by which 27HC may impact breast cancer by examining refined measures of body fat distribution in relation to 27HC levels; (3) it examines the relationship between expression of 27HC- regulating enzymes and clinicopathologic characteristics in breast tumors to interrogate the biological mechanisms mediated by 27HC; (4) it leverages the high quality resources within the MEC (e.g. extensive questionnaire information, blood and tumor tissue samples, and >20 years of follow-up data). The findings from this proposal will expand our understanding of the role of the 27HC-estradiol pathway in breast cancer, particularly in regards to different body fat distribution. This knowledge could serve as the basis for personalized breast cancer prevention and treatment strategies across different racial/ethnic populations.